1. Field of the Invention
The present invention relates to a thin film forming apparatus for forming a thin film on a surface of a square (regular square or rectangular) substrate.
2. Description of the Related Art
Conventionally, in the field of a thin film forming apparatus to which CVD (Chemical Vapor deposition) apparatus is applied, there is a demand to form a thin film of uniform film thickness and uniform film characteristic at low temperature so that the use of the thin film forming apparatus can be extended.
In order to meet the demand, the present applicant has already invented a thin film forming apparatus disclosed in JP-A(Japanese Unexamined Patent Publication) No. 5-55194.
According to the conventional thin film forming apparatus, plasma of high frequency is made of raw material gas in a film forming chamber; ion beams are drawn out from an ion source into the film forming chamber; both plasma and a surface of a substrate are irradiated with the ion beams; and a thin film of uniform thickness and uniform characteristic is accumulated and formed at low temperature on the substrate by the effect of migration of radicals in the plasma generated by the irradiation of the ion beams and also by the control of film quality.
In the same manner as that of the other existing apparatus such as a plasma CVD apparatus, in this thin film forming apparatus to which this type CVD apparatus is applied, a high frequency electrode for generating plasma of high frequency in the film forming chamber is cylindrical, and plasma of high frequency, the profile of which is spherical, is formed in a plasma generating region inside the electrode.
In this case, a cross-section of plasma of high frequency, which is parallel to the substrate, is circular. Concerning a disk-shaped substrate such as a silicon wafer, when a diameter of the high frequency electrode 1 is appropriately determined as shown in the plan view of FIG. 13, an overall surface of the circular substrate 2 is appropriately covered with plasma of high frequency, so that a thin film of uniform thickness and uniform characteristic can be effectively formed on the surface of the substrate 2.
However, concerning the square substrate, the surface of which is formed into a regular square or a rectangle, such as a glass substrate used for a liquid crystal display panel of TFT-LCD(Thin Film Transistor-Liquid Crystal Display Device), it is impossible to effectively utilize high frequency electric power and form a thin film of uniform thickness and uniform characteristic, which will be explained below.
That is, when it is attempted that a thin film of uniform thickness and uniform characteristic is formed on an overall surface of a square substrate by a conventional apparatus, it is necessary that a diameter of the high frequency electrode is set at a value longer than a diagonal line of the substrate so that plasma of high frequency can cover the overall surface of the substrate.
Even in the case of a regular square substrate, a hatched portion of plasma of high frequency, which protrudes from the substrate 2 as shown in FIG. 14(a), does not contribute to the formation of a film. Therefore, high frequency electric power is wasted, and it is impossible to enhance the film forming efficiency.
In order to prevent the film surface from being stained with a redundant thin film when the redundant thin film, which has been attached to a portion except for the substrate, is peeled off, it is necessary to conduct cleaning frequently, and the manufacturing cost is necessarily raised.
In the case of a rectangular substrate, a hatched portion of plasma of high frequency protruding from the substrate 2 is more increased than the case of a square substrate. Therefore, the film forming efficiency is further deteriorated, and it becomes necessary to conduct cleaning more frequently.
Consequently, the conventional apparatus is disadvantageous in that it is impossible to form a thin film of uniform thickness and uniform characteristic on the overall surface of the square substrate effectively.
In the conventional apparatus, ion beams are drawn out from an ion source into a film forming chamber so that a circular portion of the surface of the substrate can be irradiated with the ion beams.
Therefore, in the same manner as that of the above plasma of high frequency, when it is attempted that the overall surface of the square substrate is irradiated with ion beams, a waste portion of plasma of high frequency, which is not irradiated onto the substrate, is increased. From this viewpoint, it is impossible to effectively form a thin film of uniform thickness and uniform characteristic on the substrate.
When it is attempted that a thin film of uniform thickness and uniform characteristic is formed by the above type thin film forming apparatus, a diameter of the high frequency electrode must be larger than the size of the substrate. Therefore, it is impossible to reduce the size of the thin film forming apparatus.
When an area of the substrate is increased, an electric field of high frequency becomes nonuniform and unstable because of a delay of transmission of high frequency caused by an increase in the size of the high frequency electrode, and it becomes difficult to form plasma of high frequency uniformly. Therefore, a failure in thin film formation tends to occur.